Electric discharge devices

ABSTRACT

An electric discharge device has a cold cathode of solid, easily vaporizable material, and a triggering arrangement comprising a dielectric member, disposed near the cathode and anode, and having on one side a sputtered film of easily vaporizable material and on the other side a trigger electrode. The device operates at high vacuum, and a discharge can be triggered by applying a voltage pulse to the trigger electrode, causing vaporization of material from the film. For example, the device may be a switching device, a travelling-wave device, or an ion propulsion device.

United States Patent 1 1111 3,758,803

Cook et a1. 1 1 Sept. 11, 1973 154] ELECTRIC DISCHARGE DEVICES 3,465,192 9/1969 Lafferty 313/173 X 3,331,981 7/1967 L ffert [75} Inventors: Kenneth George Cook, Northwood; 3,394.28] 7/968 3 n t 4 Robert J p Wheldon, Chalfont, 3,243,640 3 1966 Eichenbaum 315/35 St. Peter, both of England 73 Assignee: The M-O Valve Company Limited, Primary ExaminerRudolph Rolinec London, England l t gant Examiner Saxt1eld Chatmon,Jr. Attorney- Morris Kirschstein. Bertram Ottinger et al. [22] Filed: Feb. 18, 1972 [21] Appl. No.: 227,392 [57] ABSTRACT An electric discharge device has a cold cathode of 521 U.S. Cl 313/174, 315/35, 315/330, Solid easily vapofilable material, and a triggering 313/178, 313/181, 313/198, 313/201 rangement comprising a dielectric member, disposed 51 Int. Cl. H0lk 1/50 the cathode and anode and having one Side a [58] Field of Search 3 1 3/174, 178, 179, sputtered film of easily vaporilable material and on the 313/181, 198, 201; 315/35, 330, 335 other side a trigger electrode. The device operates at high vacuum, and a discharge can be triggered by ap- 56] References Cited plying a voltage pulse to the trigger electrode, causing UNITED STATES PATENTS vaporization of material from the film. For example, the device may be a switching device, a travelling-wave 3,087,092 4/1963 Lafferty 313/178 X device or an ion propulsion device 3,255,376 6/1966 Elchenbaum... 3l5/3.5 3,611,008 10/1971 Anderson 313/178 5 Claims, 3 Drawing Figures Generator Pulse and cathode having on one side a film of discrete separated microscopic particles of easily vaporizable material, said particles being of different sizes, and on the other side, electrically insulated from said film, a trigger electrode, whereby, when the anode and cathode are. maintained in a vacuum, and when a suitable voltage is applied between the anode and the cathode, application of a suitable voltage pulse between the trigger electrode and the particles of said film causes vaporization of material from said film so as to cause a discharge to be initiated between the anode and cathode.

By an easily vaporizable material is meant a material having a latent heat of vaporization less than 50 kilocalories per mole, and a vapor pressure at l000 C greater than 0.l torr. By a vacuum is meant a pressure of less than ltorr.

- Said particles may, for example, have diameters in the range 2-10 microns, and be spaced apart by-similar distances from each other.

Generally, a device in accordance with the invention will comprise an evacuable envelope completely enclosing the anode and cathode so as to allow said vacuum to be produced. Said member of dielectric material may then comprise part of the envelope.

However where the device is intended to operate in an evacuated environment, e.g., in the case where the device is an ion propulsion device for use in the vacuum of outer space, no such envelope is required.

Conveniently, said film is formed by sputtering of material from the cathode in an arc discharge.

Another object of the invention is to provide a novel form of electric discharge device for producing an electron or ion beam. Y

Thus, according to another aspect of the invention an electric discharge devicecomprises an anode, a cold cathode of a solid, easily vaporizable material, the arrangement being such that when the anode and cathode are maintained in a vacuum and when a suitable voltage is applied between the anode and cathode a discharge can be initiated between the anode and cathode, sustained by a plasma formed from material vaporized from the cathode, and means for extracting ions or electrons from said plasma to form a beam of ions or electrons as the case may be.

Said means for extracting ions or electrons conveniently comprises an accelerating electrode adapted to pulsion device, for use in the vacuum of outer space, no such envelope is required.

Said easily vaporizable material may comprise, for example, cadmium, zinc, bismuth or magnesium.

Three electric discharge devices in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which:

FIG. 1 is a schematic sectional elevation of a switching device;

FIG. 2 is a schematic sectional elevation of a part of a microwave device; and

FIG. 3 is a schematic sectional elevation of an ion propulsion device.

Referring to FIG. 1, the switching device comprises a cup-shaped glass envelope 1. A cathode 2 is disposed within the envelope 1, and comprises a solid mass of cadmium at the base of theenvelope. Also disposed within the envelope is a cup-shaped metal anode 3, supported by means not shown, and spaced approximately 2mm from the cathode 2. Anode and cathode leads, 4 and 5, sealed through the wall of the envelope 1, are provided for making external electrical connection to the anode and cathode.

The envelope 1 is evacuated to a pressure of less than 10 torr. 7

A thin film 6 of cadmium is formed on the inner surface of the envelope 1 by sputtering cadmium from the cathode 2 in an arc discharge. The film 6 is not continuous, microscopic examination showing it to consist of a large number of discrete particles, some relatively large, and others relatively small. Typically, said particles have diameters in the range 2-10 microns and are spaced apart by similar distances. The film 6 is in electrical contact with the cathode 2, and extends up the inner surface of the envelope to a region where it lies adjacent the outer surface of the anode 3. The gap between the film and the anode in this region is approximately lmm.

An external trigger electrode 7 is formed on the outside surface of the envelope 1, in the form of a band completely encircling the envelope, in the region of the lmrn spacing between the film and the anode.

In operation of the device, when a voltage, for example of the order of [00 volts, is applied between the anode and cathode, a discharge can be initiated between the anode and cathode by applying a trigger pulse, typically of magnitude 2 kilovolts and with a rate of rise of the order of l kilovolt/microsecond, between the trigger electrode 7 and the cathode 2. Typically, the capacitance between the trigger electrode 7 and the cathode 2 is of the order of 10 picofarads, so that when the trigger pulse is applied capacitative current of the order of 10 milliamps flows between the trigger electrode and cathode, via the sputtered film 6.

Consideration of the capacitances between the trigger electrode 7 and the sputtered film 6, and between thediscrete particles of the film 6, suggests that when the trigger pulse is applied a voltage stress is set up between the discrete partgicles, in particular between large and small particles. It is throught that this stress initiates field emission with sufficient dissipation of energy in the film to cause vaporization of cadmium from the film. This vaporized material produces a plasma, which initiates the discharge between the anode 3 and cathode 2.

It will be seen that the device is effectively a vacuum valve before triggering, but after triggering, due to vaporization of cadmium from the cathode 2 and the film 6, acts effectively as a gas-filled device.

Because of the very small spacing between the anode 3 and the cathode 2 and film 6, the triggering of the discharge is very rapid, being sufficiently fast to support a current pulse rising in less than I microsecond. However, since the device is effectively a vacuum valve when it is not triggered, the hold-off voltage can be in excess of kilovolts. Thus, the device has a potential use as a pulse modulator.

Since the device does not have a gas filling, the problem of so-called gas clean up does not arise. In addition, the device requires no heater power, starts virtually instantanesouly, and can operate over a wide range of ambient temperatures. Since the cathode is solid, the device can operate in any position.

Referring now to FIG. 2, the microwave device includes an elongated evacuated glass envelope 11, at one end of which is an electron gun, comprising: a solid cadmium cathode 12, a metal anode 13, aninternal sputtered cadmium film 14, of discrete microscopic particles and an external trigger electrode 15, similar to the corresponding parts in the device in FIG. 1. As in the case of FIG. 1', in operation, a discharge can be initiat ed between the anode l3 and the cathode 12 by application of -a trigger pulse between the electrode 15 and the cathode 12. When a discharge is passing, a cadmium plasma is formed between the anode and the cathode.

Electrons can be extracted from this plasma, via an In this way, there is formed a beam 18 of electrons,

typically with energies of the order of 2-20keV. This electron beam is passed down the axis of the envelope 11, to an electron collector 19, which is held at the same potential as the electrode 17.

Between the gun and the collector 19, the electron beam passes through travelling wave tube structure, comprising a helix 30,'surrounded by a solenoidal magnet 31 which serves to collimate the electron beam 18. A microwave signal introduced into an input waveguide 32 is coupled into one terminal 33 of the helix, and travels down to the other terminal 34 of the helix, where it is coupled into an output waveguide 35. The field produced by the wave travelling down the helix interacts with the electron beam 18 and, provided the phase relations are correctly chosen, the wave absorbs energy from the electron beam, and hence is amplified.

Travelling wave tube structures are, of course, known per se, and the'details of their design and operation are therefore not discussed here.

Such a device starts virtually instantaneously, and requires no heater power.

In a modification of the device shown in FIG. 2, the travelling wave structure may be replaced by a known slow wave structure.

Referring now to FIG. 3, the ion propulsion device comprise a glass envelope 2], a solid cadmium cathode 22, a metal anode 23, having an aperture 24, an internal sputtered cadmium film 25 of discrete microscopic particles, an external trigger electrode 26, and a tubu- 4 lar accelerating electrode 27, similar to the corresponding parts of the device in FIG. 2.

In contrast to the case of FIG. 2, however, the envelope 21 has an opening 28 in its end remote from the cathode. The ion propulsion device is intended to operate in the vacuum of outer space, so that in operation, the inside of the envelope is at a suitably low pressure. Thus, as in the case of FIG. 2, in operation, a discharge can be initiated between the anode 23 and the cathode 22 by application of a trigger pulse between the electrode 26 and the cathode 22, so as to form a cadmium plasma.

Positively charged cadmium ions are extracted from the plasma, and accelerated to form an ion beam 29 of high energy, typically of the order of SkeV, by means of the accelerating electrode 27, to which is applied a negative potential with respect to the anode 23, the ions being ejected from the device through the opening 28. In this way, a thrust is produced, in the opposite direction to that in which the ions are accelerated, which thrust can be used to propel or maneuver a space vehicle.

Such a device has the following advantages over known hot cathode mercury ion engines:

a. No separate power is required to vapourise propellant.

b. No hot cathode is required.

0. No magnet is needed.

a d. No close control of pressure is required between inside of device and outside (space vacuum).

What we claim is:

.1. An electric discharge device comprising:

an anode;

a cold cathode of a solid easily vaporizable material spaced from and electrically insulated from said anode;

means for applying a positive voltage to the anode relative to the'cathode;

a dielectric member disposed in the region of the anode and the cathode,

having on one side a film of discrete microscopic particles' of easily vaporizable material,

said particles being spaced apart from each other and being of different sizes, and

having on the other side, electrically insulated from and substantially parallel to said film,

a trigger electrode, said trigger electrode being capacitively coupled to said film via said dielectric member;

means for applying a voltage pulse between the trigger electrode and the particles of said film;

said film being positioned to allow material vaporized from the particles thereof to form a plasma in the region of said anode and cathode.

2. An electric discharge device according to claim 1 wherein said film is formed by sputtering of material from said cathode in an arc discharge.

3. An electric discharge device according to claim 1, further including an evacuable envelope completely enclosing the anode and cathode for maintaining a vacuum in the region of the anode and cathode.

4. An electric discharge device according to claim 1 wherein said member of dielectric material comprises part of said envelope, said film of discrete particles being formed on an inner surface of said envelope adjacent said anode, and said trigger electrode being 3,758,803 6 formed on an outer surface of said envelope adjacent from the cathode when a discharge is passing between said film.

5. An electric discharge device according to claim' 1 further including means for extracting charged partiucles' cles from a plasma formed from material vaporized 5 I anode and cathode to form a beam of said charged par- 

1. An electric discharge device comprising: an anode; a cold cathode of a solid easily vaporizable material spaced from and electrically insulated from said anode; means for applying a positive voltage to the anode relative to the cathode; a dielectric member disposed in the region of the anode and the cathode, having on one side a film of discrete microscopic particles of easily vaporizable material, said particles being spaced apart from each other and being of different sizes, and having on the other side, electrically insulated from and substantially parallel to said film, a trigger electrode, said trigger electrode being capacitively coupled to said film via said dielectric member; means for applying a voltage pulse between the trigger electrode and the particles of said film; said film being positioned to allow material vaporized from the particles thereof to form a plasma in the region of said anode and cathode.
 2. An electric discharge device according to claim 1 wherein said film is formed by sputtering of material from said cathode in an arc discharge.
 3. An electric discharge device according to claim 1, further including an evacuable envelope completely enclosing the anode and cathode for maintaining a vacuum in the region of the anode and cathode.
 4. An electric discharge device according to claim 1 wherein said member of dielectric material comprises part of said envelope, said film of discrete particles being formed on an inner surface of said envelope adjacent said anode, and said trigger electrode being formed on an outer surface of said envelope adjacent said film.
 5. An electric discharge device according to claim 1 further including means for extracting charged particles from a plasma formed from material vaporized from the cathode when a discharge is passing between anode and cathode to form a beam of said charged particles. 